In 1955 Ed Lorenz derived a diagnostic of atmospheric energetics based on the Available Potential Energy (APE). This diagnostic has been widely used to understand the circulation of the atmosphere. It turns out that the concept of APE arises naturally in geophysical fluid dynamics, and has numerous extensions. This all becomes clear when things are viewed within the context of Hamiltonian dynamics and pseudoenergy. For example, for vorticity dynamics the concept of "available enstrophy" is a natural extension of APE. In his 1995 paper, Lorenz noted that a limitation of his definition of APE is that the reference state is required to be at rest, so that any constraint provided by angular momentum conservation does not enter the theory. In particular, a dynamically stable flow would have non-zero APE, yet none of this APE could be spontaneously released. This is particularly an issue in the middle atmosphere, where strong zonal flows exist. A general theory of APE can be derived which includes angular momentum constraints and overcomes this problem.